This invention relates generally to switched-mode power supplies and is particularly directed to an improved AC line isolation and noise filtering arrangement in a switched-mode power supply.
In a switched-mode type of power supply, the series arrangement of a primary winding of a transformer and a controlled switch is connected to the terminals of an input voltage source, and the switch is periodically rendered conductive and in the cut-off state in an alternating manner. The duration of the conducting state of the switch is adjustable for controlling the output voltage. A rectifier is typically connected to a secondary winding of the transformer with its conduction direction established with respect to the winding sense of the secondary winding such that the rectifier conducts during the cut-off period of the switch.
Switched-mode power supplies of the type described above are used in a wide range of electronic apparatus such as television receivers and computer terminals. In an effort to maintain the AC input line unaffected by these increasing numbers of switched-mode power supply driven devices, strict limitations have been imposed upon the current which such devices may return to the AC power line. In order to reduce current leakage for safety purposes, the typical switched-mode power supply incorporates a filter arrangement having a low capacitance value. While large capacitors are more effective for filtering out higher frequency signals, i.e., on the order of the switching rate of the switching devices within the power supply, these large capacitance filters contribute to high leakage currents at the AC line frequency. Therefore, these lower frequency, leakage currents are typically fed back via ground in the switched-mode power supply. Prior art switched-mode power supplies have been unable to reconcile these competing and previously mutually exclusive operating criteria of leakage current isolation for improved safety and effective noise filtering for reduced AC line interference.
The present invention resolves the problems encountered in the prior art by providing a switched-mode power supply arrangement which provides improved high voltage isolation at low signal frequencies while filtering out higher frequency signals for minimizing current leakage to the AC line. The switched-mode power supply of the present invention provides a low impedance return path for high frequency signals in isolating them from the AC power line and also provides low capacitive value for isolation of the AC line voltage for improved safety.